Why does this simple MySQL query not return the row?

Question

I have a row in the table users with the username test . For some reason, though, this query returns an empty result set.

SELECT `id` FROM `users` WHERE `username` = "test" AND `id` != null;

However, if I remove the `id` != null segment, the query returns the result id = 1 .

But 1 != NULL . How is this happening?

The id field is non-nullable and is auto-increment.

Thanks!

Answer 1

Your method of checking for NULL is probably the issue. In MySQL, try the following:

SELECT `id` FROM `users` WHERE `username` = "test" AND `id` IS NOT NULL;

To check for NULL and an empty string, you can use:

SELECT `id` 
FROM `users` 
WHERE `username` = "test" 
AND (`id` IS NOT NULL OR `id` != "");

Answer 2

The query doesn't return a row because the predicate " id != NULL " will never return TRUE.

Th reason for this is that boolean logic in SQL is three valued. A boolean can have values of TRUE , FALSE or NULL .

And an inequality comparison will return NULL whenever one (or both) of the values being compared is NULL .

The SQL standard means to compare to a NULL is to use id IS NULL or id IS NOT NULL . MySQL also adds a convenient null-safe comparison operator which will return TRUE or FALSE:

col <=> NULL . Or, in your case NOT (col <=> NULL)

Answer 3

try using IS NOT NULL

SELECT `id` FROM `users` WHERE `username` = "test" AND `id` IS NOT NULL

Have a look at the difference

SQL Fiddle DEMO

Answer 4

By database definition in general ,Null is nothing and cannot be equated or compared with any other value. Hence ID=NUll or ID!=null wouldn't work.

Answer 5

Try:

SELECT `id` FROM `users` WHERE `username` = "test" AND `id` != '';

OR

SELECT `id` FROM `users` WHERE `username` = "test" AND `id` iS NOT NULL;